Military post offices on the Salonica front. Double weight cover fr. with 5x5c. (underpayment of 15c. double weight foreign letter rate) cancelled by pen and posted from "VOJNA POSTA*Br.999*26.5.18" with Serbian censorship "31 PREGLEDALA/VOJNA CENZURA" in violet and French "OUVERT par lAUTORITE*418" and tape, via "CHALET-A-GOURET*16.VII.18", arr. "LAUSANNE*16.VII.18". On arrival taxed with 30c. Swiss postage due (double the deficiency) and cancelled with "LAUSANNE*17.VII.18" for forward to Chailly village, Lausanne. There is another postage due stamp underneath the 30c, perhaps because of the forwarding.